[scroll area] Fix remount thumb size

[atomiks]

• I have followed (at least) the PR section of the contributing guide.

Fixes #4103
Fixes #3829

Preview: https://deploy-preview-4107--base-ui.netlify.app/experiments/scroll-area/tabs-scroll-area

For inset scrollbars, they are hidden (unless keepMounted is specified) before hydration.
This PR SSR: https://deploy-preview-4107--base-ui.netlify.app/experiments/scroll-area/scroll-area-inset
Master SSR: https://master--base-ui.netlify.app/experiments/scroll-area/scroll-area-inset

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/@base-ui/react@4107
  

  pnpm add https://pkg.pr.new/@base-ui/utils@4107
  

commit: 638e76e

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+416B(+0.09%)	🔺+159B(+0.11%)

Details of bundle changes

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[atomiks]

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